In recent years, it is generally known that, in a system (hereinafter referred to as a “network boot system”) that boots an operating system (hereinafter referred to as an “OS”) over a network, in order to reduce a load on a network boot server, a read-only cache (hereinafter referred to as a “read cache mechanism”) is provided in a part of a physical storage device on a client terminal side.
In Patent Document 1, a network boot system is disclosed in which a read cache mechanism is provided that can reduce network access to a network boot server by caching a virtual disk of the network boot server in a local disk of a PC (client terminal) that is used by an end user in the network boot system. The virtual disk is generation-managed using revise information (hereinafter referred to as “revision”) and the revision is updated along with revise of the virtual disk such as installation of an updated program and a configuration change. The read cache mechanism has a cache management table corresponding to the revision of the virtual disk in each terminal and presence or absence of cache data is decided using the management table.
The above-described network boot system further has a write cache region for caching write information and a write management flag. Due to the write management flag, whether or not data in a certain sector is modified can be decided and the read cache mechanism avoids using cache data held in a sector that is modified. As a result, the read cache mechanism effectively utilizes cache data, thereby reducing the network load. Therefore, in such a network boot system, even when a large number of client terminals are booted all at once, on the second or subsequent boot, there is almost no access to the network and the client terminals can be booted at high speeds.
The above-described network boot system has a share mode (standard mode) and a private mode. The share mode is a mode in which a plurality of client terminals share and use a virtual disk on the network boot server and is a mode for causing the system to operate in a normal operation environment. On the other hand, the private mode is a mode in which only a specified client terminal can directly write to the virtual disk on the server and is a mode for causing the system to operate in a maintenance environment. By booting a client terminal in a state of the private mode, data of the virtual disk can be modified by performing update or new installation of the OS or a program, a configuration change, or the like.